A relatively new kind of actuator, called a servomotor actuator, may replace a pneumatic cylinder commonly found on resistance spotwelding machines. It is useful to understand the prior art of resistance spotwelding and in particular the apparatus used to do resistance spotwelding. Reference is made to Chapters 17 and 19 of the eighth edition of the WELDING HANDBOOK.
The difference between a conventional resistance spotwelding welding machine and a servomotor-actuated welding machine is the means by which force is applied to the welding electrodes. Instead of a pneumatic cylinder to supply the force, an electric servomotor is used. In order to supply the required force, on the order of 1000 LBS, with a motor small enough to be practical, the servomotor is usually used to drive gearing in the form of a screw or a nut which then supplies the force to the welding electrodes.
The servomotor is controlled by an electronic servo drive, which regulates the current into the motor to regulate its torque and thus the clamping force on the welding electrodes. In addition to controlling torque (and therefore electrode force), the electronic servo drive also measures the position of the motor and/or the screw it drives so it can control both motor speed and position. Thus, a servomotor-actuated welding machine has the capability to position its welding electrodes precisely anywhere between fully closed and fully open positions. In addition, a servomotor-actuated welding machine has the capability to move the welding electrodes at a particular velocity and to measure changes in force as a function of position.
As illustrated in FIG. 2, a servo system traditionally includes a compensator which takes the desired response (usually in terms of a position or velocity of a motor) and compares it to that measured by a sensing system. Based on this measurement, a new input for the motor is calculated. The compensator may be implemented with either analog or digital computing hardware. Since the compensator consists of computing electronics, it rarely has sufficient power to drive the motor directly. Consequently, the compensator's output is used to command a power stage. The power stage amplifies that signal and drives the motor. There are a number of motors that can be used in servo systems such as DC motors, brushless DC motors, and hybrid stepping motors. To achieve a mechanical advantage, the motor shaft is usually connected to gearing or gears that move the actual load. The performance of the motor is measured by either a single sensor or multiple sensors. Two common sensors, a tachometer (for velocity) and an encoder (for position), are shown. Finally, those measurements are fed back to the compensator.
The servomotor-actuated resistance welding machine itself is not an object of this invention and little explanation of its internal operation is given herein. For information on how a servomotor-actuated resistance spotwelding machine works, refer to U.S. Pat. Nos. 4,670,641; 5,340,960; 5,405,075 and 5,742,022. These patents discuss the operation of a resistance spotwelding gun, which is a type of resistance spotwelding machine. The general principles described therein apply to all types of resistance spotwelding machines.
It is usually desirable, for economic reasons, to speed up the resistance spotwelding process as much as possible. A significant portion of the time required for welding is called "Squeeze time". As the welding gun is closed on the workpiece, squeeze time is the time interval allowed for the welding gun to close and build up force on the welding electrodes. When a pneumatic cylinder is used to supply the force on the welding electrodes, the squeeze time can vary widely due to changes in the incoming plant air supply pressure. Also, it is difficult to measure exactly when the force on the welding electrodes has actually reached the proper value. Friction and inertia of the cylinder and welding gun make measurements of air pressure in the cylinder unreliable to determine when welding force has reached a proper value. A load cell to measure the actual force on the welding electrodes is expensive, fragile, and unreliable.
Therefore, usual practice is to use a fixed squeeze time to allow the welding electrodes to close and attain proper welding force. If this time is set too low, inconsistent welds result. If it is set too high, production speed is slowed.
As many spot welds are made, wear on the welding electrode faces occurs. Depending upon the material being welded, this wear occurs in various ways. With welding materials such as aluminum, the faces of the electrodes contacting the weld area wear down, and due to the generally conical or spherical shape of the welding electrodes, the contact area of the electrodes to the work increases. As this happens, the current and force required for an optimum weld increase, since the larger contact area spreads both the current and applied force over a larger area.
In other material, such as galvanized steel, welding electrode wear occurs differently. Since the zinc coating of galvanized steel has an affinity for copper and the temperature required for welding steel is considerably above the melting point of zinc, the zinc on the part in the weld area forms an alloy (brass) on the surfaces the copper welding electrode which contact the part being welded. As the first few welds are made with new electrodes, the electrodes actually get slightly longer (they "grow" by a few thousandths of an inch) as this zinc accumulates and alloys with the copper. As more welds are made and the brass layer gets thicker, the brass on the faces of the welding electrodes is softened by the heat of the welding process and extrudes out to the sides of the contact faces of the welding electrodes.
This has the effect of both increasing the contact area of the electrodes on the surface of the part and wearing the electrode down. Both of these effects increase the current and force required for an optimum weld.
The usual method of compensating for electrode wear is with a "weld current stepper". A weld current stepper is a feature of the welding control wherein the welding current is increased (or, in special cases decreased) to compensate for welding electrode wear and deterioration. One way to implement a weld current stepper is to have the control keep track of the number of welds made and increase (or in some special cases, decrease) the welding current according to the number of welds made. Another method of implementing a weld current stepper is to use electrical measurements to identify events during the welding process and increase or decrease the welding current in response to these events. See U.S. Pat. Nos. 4,104,724; 4,885,451; 5,083,003; 5,386,096 and 5,449,877 for more information in this area.
The present methods of implementing a weld current stepper all have drawbacks and shortcomings. The rate at which welding electrodes wear or deteriorate varies tremendously due to variations in the metal being welded, variations in the electrodes themselves, adequacy of cooling water to the electrodes, and a host of other factors. From one day to the next, the rate at which welding electrodes deteriorate or wear can change by a factor of more than 2. Methods of simply counting welds are clearly inadequate to compensate for these variations. Other methods which measure electrical phenomena to implement a weld current stepper base decisions on effects caused partially by welding electrode wear and partially by other factors, such as material variations. These other factors cause the weld current stepper to respond to things other than wear or deterioration of the welding electrodes, which results in increased process variation.
If a weld is made at the very edge of a part, and only a small portion of the electrode faces are actually in contact with the part, severe weld expulsion will occur and the faces of the welding electrodes may be damaged. During this weld, the electrical measurements taken by the welding control can identify weld expulsion (See U.S. Pat. No. 4,885,451 for an explanation of how this is done).